The invention concerns a method in calendering, wherein the web to be calendered runs through one or several pairs of rolls and one roll in each pair of rolls is a heated roll, and the other roll in each pair of rolls is a roll provided with a resilient coating.
The invention further concerns a roll for use in the method as well as the use of the method and of the roll.
In the crude state, the paper coming out of a paper machine has a rough face, which requires finishing for most purposes of use, said finishing resulting in smoothing and densification of the face. For the purpose of finishing, smoothing devices (e.g. machine calenders) and resilient-nip calenders (e.g. soft or supercalenders) are known. The smoothing devices comprise hard rolls only, and they even the boundary faces of the paper so that the parts that form the surface of the paper are substantially in one plane. The roll nips in a resilient-nip calender are so-called soft roll nips, i.e., nips in which a hard roll forms a pair of rolls with an elastically resilient roll. At present, the elastically resilient rolls in resilient-nip calenders are entirely predominantly paper rolls, i.e., rolls that consist of paper strips fitted as layers one above the other. To a certain extent, a calender also affects the smoothing, but in the first place, however, the glaze, i.e., the surface of the paper web is densified and closed.
The designations supercalendering and softcalendering used in the application are not official designations; instead of softcalendering some people speak of matte-calendering. A supercalender is an off-machine device, whereas a softcalender is either an on-machine (1 or 2 nips) or an off-machine (at least 4 nips) device. In softcalenders the resilient rolls are not paper rolls, which is the case in supercalenders, but they are different types of polymer or equivalent rolls, in which their own internal generation of heat is inferior to that taking place in paper rolls and in which the susceptibility of marking is lower. In its purest form softcalendering would be calendering that is carried out as on-line operation by making use of high temperatures (significantly higher than in a supercalendar) with a minimum number of nips. At present a softcalender is used extensively instead of a machine calendar with mat qualities as well as with coated papers in connection with a paper machine or coating machine when either the running speeds are low and/or the machines are narrow and/or the linear loads and/or the temperatures used are not among the highest. As a rule, high-glaze papers continue to be calendered by means of a supercalender.
A module calender refers to a calender which consists of one or several nips (i.e., modules) formed by a steel roll and a soft roll as described in the patent.
Most resilient-nip calenders are so-called supercalenders, which consist of a number of rolls fitted one above the other, said rolls being alternating soft and hard. In this ways the paper web runs through a number of roll nips, through one nip after the other. The hard rolls used in a typical supercalender are of metal, as a rule of steel and/or cast iron, and the soft rolls are paper- or cloth-filled.
The metal rolls in the pairs of rolls are usually heated so as to obtain good results.
The main problem associated with paper calendering has been the poor ability of the resilient rolls to endure high temperatures. Prior-art attempts to extend the service lives of resilient rolls involved cooling of the steel core of the roll by making its filler material of asbestos and cellulose and by using so-called heat-resistant fibers. For example, according to the U.S. Pat. No. 3,291,039, a resilient roll with superior resistance to heat was made of several sheets of cellulose fibers which had been treated with an additive so that they should endure highest temperatures and so that the coating consisted of several layers. According to the patent, this roll endures temperatures of 110.degree.-140.degree. C.
As prior art, reference is also made to the U.S. Pat. No. 3,451,331, wherein a calender is used that is provided with heated hard steel rolls, as well as to the Finnish patent application 864020.
In types of supercalendering mentioned above, at the side of the resilient roll the properties of the paper are developed more weakly than at the side of the steel roll. One reason is the different surface properties of the rolls, and the other reason is that the surface temperatures of the steel rolls are higher than those of the resilient rolls. When moving over to softcalendering, attempts are made to employ even higher temperatures, whereby the difference between the sides is increasing further. Thereat, in order to minimize unequal-sidedness, in softcalendering it is necessary to use an even number of nips, whereby both sides of the paper receive an equal number of steel-roll treatments, for without a resilient counter-roll it is impossible to calender a paper to high quality.
The difference in the calendering result given by a steel roll and by a resilient roll and the one-sidedness of the use of high temperatures (only at the side of the steel roll) constitute a problem both in supercalendering and in softcalendering.
Thus, in prior art, to solve this problem, in softcalendering an even number of nips was used, in which case both sides of the paper receive an equal number of treatments with a (hot) steel roll.
Since it has not been possible to make use of heat at more than one side of the paper per nip, this involves extra nips and a loss of bulk of the paper.